The present invention relates to measurement techniques of sensitive quantum systems operating in the microwave domain, such as superconducting quantum circuits, which require for their measurement, high gain, low-noise output chains that also provide sufficient protection to the measured systems against noise and more specifically, to a high fidelity and high efficiency qubit readout scheme.
In one approach called circuit quantum electrodynamics, quantum computing employs nonlinear superconducting devices called qubits to manipulate and store quantum information, and resonators (e.g., as a two-dimensional (2D) planar waveguide or as a three-dimensional (3D) microwave cavity) to read out and facilitate interaction among qubits. As one example, each superconducting qubit may comprise one or more Josephson junctions shunted by capacitors in parallel with the junctions. The qubits are capacitively coupled to 2D or 3D microwave cavities. The electromagnetic energy associated with the qubit is stored in the Josephson junctions and in the capacitive and inductive elements forming the qubit. To date, a major focus has been on improving lifetimes of the qubits in order to allow calculations (i.e., manipulation and readout) to take place before the information is lost due to decoherence of the qubits. Currently, the coherence times of superconducting qubits can be as high as 100 microseconds, and efforts are being made to increase their coherence times.